Cellulose fibers having low water retention value and low capillary desorption pressure

ABSTRACT

The present invention provides cellulose fibers having low median desorption pressures and low water retention values (WRV), which exhibit improved drainage and fluid flow properties. These fibers are particularly well suited for use in acquisition, distribution, and acquisition-distribution layers, or in absorbent core structures. One embodiment of the invention is a method for preparing cellulose fibers by refining cellulose fibers to a freeness ranging from about 300 to about 700 ml CSF and crosslinking the refined fibers. Another embodiment of the invention is fibers crosslinked with at least one saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl discarboxylic acid, bifunctional monocarboxylic acid, or amine carboxylic acid. A crosslinking facilitator, such as oxalic acid, may be present during the crosslinking reaction to improve the efficacy of the crosslinking agent. Yet another embodiment of the invention is an absorbent core comprising SAP particles and reversible crosslinked fibers.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/247,078, filed Nov. 10, 2000, and U.S. ProvisionalApplication No. 60/286,298, filed Apr. 25, 2001, both of which arehereby incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to cellulose fibers having low waterretention values and low median desorption pressures, as measured in acapillary absorption-desorption cycle, methods for preparing thesefibers, and-absorbent structures containing these fibers.

BACKGROUND OF THE INVENTION

[0003] Absorbent structures are important in a wide range of disposableabsorbent articles including infant diapers, adult incontinenceproducts, sanitary napkins and other feminine hygiene products and thelike. These and other absorbent articles are generally provided with anabsorbent core to receive and retain body liquids. In a conventionalabsorbent structure, the absorbent core is placed between a liquidpervious topsheet, whose function is to allow the passage of fluid tothe core, and a liquid impervious backsheet whose function is to containthe fluid and to prevent it from passing through the absorbent articleto the garment of the wearer of the absorbent article.

[0004] An absorbent core for diapers, adult incontinence pads andfeminine hygiene articles frequently includes fibrous batts or websconstructed of defiberized, loose, fluffed, hydrophilic, cellulosicfibers. Such fibrous batts form a matrix capable of absorbing andretaining some liquid. However, their ability to do so is limited. Thus,superabsorbent polymer (SAP) particles, granules, flakes or fibers(collectively particles), capable of absorbing many times their weightof liquid, are often included in the absorbent core to increase theabsorbent capacity of the core, without having to substantially increasethe bulkiness of the core. In an absorbent core containing matrix fibersand SAP particles, the fibers physically separate the SAP particles,provide structural integrity for the absorbent core, and provide avenuesfor the passage of fluid through the core.

[0005] Absorbent cores containing SAP particles have been successful,and in recent years, market demand has increased for thinner, moreabsorbent and more comfortable absorbent articles. As core absorbencyhas improved, the ability of the core to rapidly drain fluid from theabsorbent article topsheet has become critical to maintaining a dryenvironment between the skin of the wearer of the absorbent article andthe topsheet of the article.

[0006] The ability of the absorbent core to drain fluid from the layerimmediately above it in the absorbent structure is controlled bygravity, by the number, size and spatial orientation of unoccupiedvolumes (voids or pores) in the absorbent core, and by thecharacteristics of the core components that impact fluid flow such asthe wettability of the components by the acquired fluid, as indicated bycontact angle, the surface tension of the acquired fluid and theviscosity of the acquired fluid. An acquisition layer, distributionlayer, or acquisition and distribution layer can be included in theabsorbent structure between the top sheet and absorbent core tofacilitate draining of fluid into the absorbent core.

[0007] For optimum performance of an absorbent structure in terms offluid capacity and core utilization, it is critical for fluids acquiredby the absorbent core to move quickly from the moistened regions of thecore to the dry regions of the core. The ability of the absorbent coreto move fluids rapidly from a moistened region of the core to a dryregion of the core may be described in terms of its permeabilityperformance. The permeability of an absorbent core is defined as theability of a liquid to flow through the absorbent core.

[0008] The ability of a first substrate, such as an absorbent core, todrain fluid primarily by capillary forces from a second substrate, suchas an acquisition and distribution layer, is known as the partitionproperty of the substrates.

[0009] It is known to those skilled in the art that absorbent structurescontaining absorbent cores with good fluid partition properties alsoexhibit poor fluid permeability. Similarly, absorbent structures havinggood fluid permeability exhibit poor fluid partition properties.Consequently, it is important that fibers used in an acquisition layerhave a higher degree of stiffness or resiliency (measured as drycompressibility) under the weight of the diaper wearer than conventionalfibers used in an absorbent core. This resiliency enables the interfiberspaces or voids in the acquisition layer to be maintained while thediaper is worn so that fluids can be quickly absorbed through the liquidpermeable topsheet into the absorbent structure of the diaper.

[0010] It is also important for core fibers not to densify when wet, tothe point that fluid flow into and through the absorbent core isrestricted. In addition, core fibers must have sufficient physicalintegrity to maintain separation of wet SAP particles in an absorbentcore so that as the particles swell, gel blocking is minimized oreliminated.

[0011] One method for increasing the stiffness and resiliency of fibersis by crosslinking them. Cellulose fibers can be stiffened by intrafibercrosslinks, i.e., crosslinks between two different portions of the samefiber, and to a lesser degree by interfiber crosslinks, i.e., crosslinksbetween two different fibers.

[0012] Intrafiber crosslinking with certain aliphatic and alicyclicC₂-C₉ polycarboxylic acids is disclosed in U.S. Pat. No. 5,190,563issued to Herron et al. A “C₂-C₉ polycarboxylic acid” as defined byHerron et al. is an organic acid containing two or more carboxyl groupsand from 2 to 9 carbon atoms in the chain or ring to which the carboxylgroups are attached. Suitable C₂-C₉ polycarboxylic acids contain atleast three carboxyl groups or two carboxyl groups with a carbon-carbondouble bond present at the alpha, beta position relative to one or bothcarboxyl groups. When two carboxyl groups are separated by acarbon-carbon double bond or are both connected to the same ring, thetwo carboxyl groups must be in the cis configuration. Examples of suchpolycarboxylic acids include citric acid, 1,2,3-propanetricarboxylicacid, 1,2,3,4-butanetetracarboxylic acid (BTCA), and oxydisuccinic acid.Herron et al. also found that cellulosic fibers crosslinked withaliphatic alkanes containing 4 carboxyl groups, namely, BTCA, had lowerwater retention values than those containing 3 carboxyl groups, namely,citric acid and 1,2,3-propane tricarboxylic acid. Typically, fibers withlower water retention values are stiffer than those having higher waterretention values.

[0013] In contrast to cellulosic fibers having intrafiber crosslinks,cellulosic fibers having interfiber crosslinks, such as those found inmost papers, are stiff when dry but do not necessarily maintain theirstiffness when wet. Interfiber crosslinking of paper with citric acidand 1,2,3,4-butanetetracarboxylic acid and fabrics with maleic acid,citric acid, and 1,2,3,4-butanetetracarboxylic acid is disclosed in D.F. Caulfield, TAPPIJ., 77(3): 205-212 (1994); D. Horie & C. J. Biermann,TAPPIJ., 77(8):135-140 (1994); Y. J. Zhou, P. Luner & P. Caluwe, J.Appl. Polymer Sci., 58:1523-1534 (1995); and D. D. Gagliardi and F. B.Shippee, Am. Dyestuff Reptr., 52:300 (1963).

[0014] Zhou et al., supra, studied the wet strength of paper crosslinked(interfiber) with certain polycarboxylic acids. Generally, interfibercrosslinking increases the wet strength of paper fibers. Zhou et al.found that the wet strength of the paper increased as the functionalityof the polycarboxylic acid (i.e. the number of carboxyl groups in thepolycarboxylic acid) increased. For example,1,2,3,4-butanetetracarboxylic acid (BTCA) (4 carboxyl groups) was foundto be more effective than tricarballylic acid (TCA) (3 carboxyl groups),which in turn was found to be significantly more effective than succinicacid (2 carboxyl groups). Paper treated with succinic acid exhibitedvery little wet strength.

[0015] H. J. Campbell and T. Francis, Textile Res. J., 35:260 (1965),crosslinked cotton cellulose with specific polycarboxylic acids. Thereaction was catalyzed with trifluoroacetic anhydride (TFAA),necessitating the use of a non-aqueous solvent, in this case benzene, toprevent hydrolysis of the TFAA. Campbell and Francis reported thatsuccinic acid and glutaric acid showed only slight reactivity withcotton cellulose. Furthermore, they reported that esterification (orcrosslinking) did not take place with oxalic acid. Malonic acid wasfound to react readily with cotton cellulose producing fabrics whichwere yellowed to an extent depending upon the degree of reaction.

[0016] Frequently, crosslinked cellulosic fibers are manufactured at alocation remote from where they are incorporated into absorbentstructures. Since the crosslinked fibers are bulky and have little fiberto fiber contact, they do not bond well to one another. Hence, sheetsformed from crosslinked fibers fall apart easily. As a result,crosslinked cellulosic fibers are generally shipped in bales. Thisincreases the cost of shipping the crosslinked fibers and the cost ofmanufacturing the absorbent structure. It would, therefore, be desirableto prepare sheets of cellulosic fibers containing a crosslinking agent.

[0017] A “crosslinkable” cellulosic fibrous product formed into a web orsheet is disclosed in International Publication No. WO 00/65146. Thecrosslinkable product is formed by applying a crosslinking agent to amat of cellulosic fibers and then drying the treated mat (withoutheating to a temperature sufficient to cure the crosslinking agent) suchthat substantially no crosslinking occurs and the product issubstantially free from crosslinks.

[0018] U.S. Pat. No. 6,059,924 disclose a process for enhancing the drycompression characteristics and the wicking property of fluff pulp. Theprocess includes mildly refining a chemical pulp slurry prior toformation of a fluff pulp sheet.

[0019] There is a continuing need for improved cellulose fibers thathave low water retention values and low median desorption pressures forincorporation into acquisition, distribution, andacquisition-distribution layers. There is also a need for core or matrixfibers that facilitate fluid flow into and through the absorbent coreand maintain sufficient physical integrity to minimize or eliminate thegel blocking of swollen SAP particles. Finally, there is a need formethods of preparing sheets of crosslinkable cellulosic fibers.

SUMMARY OF THE INVENTION

[0020] This invention provides cellulose fibers having low mediandesorption pressures, as measured in a capillary absorption-desorptioncycle, and low water retention values (WRV), which exhibit improveddrainage and fluid flow properties. These fibers are particularly wellsuited for use in acquisition, distribution, andacquisition-distribution layers, and in absorbent core structures.

[0021] According to one embodiment, the fibers of the present inventionare crosslinked and have a median desorption pressure, as determined ina capillary absorption-desorption cycle, of 15 cm or less. Preferably,the cellulose fibers also have a WRV of 45% or less. These fibers may beprepared by refining cellulose fibers to a freeness ranging from about300 to about 700 ml Canadian Standard Freeness (CSF) and crosslinkingthe refined fibers. According to a preferred embodiment, the fibers arecrosslinked with citric acid after refining.

[0022] Another embodiment of the invention is fibers crosslinked with atleast one saturated dicarboxylic acid, aromatic dicarboxylic acid,cycloalkyl dicarboxylic acid, bifunctional monocarboxylic acid, or aminecarboxylic acid. A crosslinking facilitator, such as oxalic acid, may bepresent during the crosslinking reaction to improve the efficacy of thecrosslinking agent. According to one preferred embodiment, the cellulosefibers are refined prior to crosslinking in order to further stiffenthem.

[0023] Another embodiment is a method of preparing crosslinkablecellulose fibers comprising the steps of (a) crosslinking cellulosefibers with at least one crosslinking agent selected from saturateddicarboxylic acids, aromatic dicarboxylic acids, cycloalkyl dicarboxylicacid, bifunctional monocarboxylic acids, and amine carboxylic acids and(b) uncrosslinking the crosslinked cellulose fibers. Preferably, thecrosslinking agent in this embodiment contains 4 carbon atoms or less.Two preferred crosslinking agents are oxalic acid and sodiumchloroacetate. The crosslinkable fibers can be formed into sheets toease their transport. Furthermore, the crosslinkable fibers can bere-crosslinked by curing the uncrosslinked cellulose fibers.

[0024] Yet another embodiment of the present invention is anacquisition, distribution, or acquisition and distribution layercomprising the cellulose fibers of the present invention.

[0025] Yet another embodiment is an absorbent core comprising cellulosefibers of the present invention. The absorbent core exhibits improvedfluid flow properties into and through the core. According to onepreferred embodiment, the absorbent core comprises SAP particles andreversible crosslinked fibers. The reversible crosslinked fibersseparate the SAP particles and provide channels for fluid flow aroundthe SAP particles from the wet to the dry areas of the absorbent core.Additionally, the reversible crosslinked fibers facilitate absorption oflarge volumes of urine (or other fluid) over a short period of time(e.g., a gush). Once urine or other fluid enters the absorbent core, thecrosslinked fibers begin to uncrosslink. The uncrosslinked fibers holdand retain the urine or other fluid to a greater extent than fibers thatare permanently crosslinked. As a result, the absorbent core hasimproved initial gush capacity compared to absorbent cores containingconventional fluff fibers and improved rewet performance compared toabsorbent cores containing permanently crosslinked fibers.

[0026] Yet another embodiment is an absorbent structure comprising theacquisition, distribution, or acquisition and distribution layer of thepresent invention and/or the absorbent core of the present invention.Preferably, the absorbent structure contains a top (acquisition,distribution, or acquisition and distribution) layer and a bottom(storage) layer in fluid communication with the top layer. The absorbentstructure exhibits superior partitioning from the acquisition and/ordistribution layer to the storage layer compared to conventionalabsorbent structures.

[0027] Yet another embodiment is an absorbent article containing theabsorbent structure of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] Definitions

[0029] The term “capillary absorption-desorption cycle” (also known asthe capillary sorption cycle or CSC) refers to the process ofdetermining the relationship between the pore volume of an absorbentstructure and capillary pressure during absorption of a liquid into theabsorbent structure and subsequent drainage of the liquid from theabsorbent structure. The capillary absorption-desorption cycle isindicative of the ability of an absorbent structure to attract, retain,and distribute fluid in the pores between the fibers of an absorbentstructure. An absorbent structure may be subjected to this cycle bysystematically lowering or raising the capillary pressure in narrowintervals, such as by the method described in the examples of thisapplication; “Capillary Sorption Equilibria in Fiber Masses”, A. A.Burgeni and C. Kapur, Textile Research Journal, 37:356-366 (1967); andP. K. Chatterjee, Absorbency, Textile Science and Technology 7, ChapterII, pp. 63-65, Elsevier Science Publishers (1985), which are herebyincorporated by reference.

[0030] The “median desorption pressure” as determined in acapillary-desorption cycle refers to the water swollen cellulose fibersability to release water. For example, a sample of cellulose fibers thatstrongly retains water exhibits a much higher median desorption pressurethan a sample of swollen cellulose fibers that readily releases water.The median desorption pressure as discussed herein is determined by themethod described in the examples of this application; “CapillarySorption Equilibria in Fiber Masses”, A. A. Burgeni and C. Kapur,Textile Research Journal, 37:356-366 (1967); and P. K. Chatterjee,Absorbency, Textile Science and Technology 7, Chapter II, pp. 63-65,Elsevier Science Publishers (1985), which are hereby incorporated byreference. This test method measures the ability of water swollencellulose fibers to retain water against hydrostatic pressure.

[0031] The “water retention value” (WRV) of a cellulose fiber may bedetermined by the methods described in TAPPI Useful Methods, UM 256, andP. K. Chatterjee, Absorbency, Textile Science and Technology Z, ChapterII, pp. 62-63, Elsevier Science Publishers (1985), which are both herebyincorporated by reference. The test measures the weight of waterremaining in a sample of water saturated cellulose fibers aftercentrifugation and expresses that quantity as a weight percent based onthe dry weight of the fibers. The WRV of a cellulose fiber is related toits drainage ability.

[0032] Any “cellulose fibers” known in the art, including cellulosefibers of any natural origin, such as those derived from wood pulp, maybe used as starting materials in the methods of the present invention.Preferred cellulose fibers include, but are not limited to, digestedfibers, such as kraft, prehydrolyzed kraft, soda, sulfite, chemi-thermalmechanical, and thermo-mechanical treated fibers, derived from softwood,hardwood or cotton linters. More preferred cellulose fibers include, butare not limited to, kraft digested fibers, including prehydrolyzed kraftdigested fibers.

[0033] Generally, cellulose fibers having thicker walls are preferablesince they are coarser and stiffer than similar fibers having thinnerwalls. The fiber walls of a fiber are defined by the lumen of a fiber(i.e. the hollow interior of the fiber) and the outer surface of thefiber. For example, since the fiber walls of southern softwoods are onaverage thicker than those of northern softwoods, fibers derived fromsouthern softwoods are preferable. More preferably, the cellulose fibersare derived from softwoods, such as pines, firs, and spruces.

[0034] Other suitable cellulose fibers include those derived fromEsparto grass, bagasse, kemp, flax and other lignaceous and cellulosicfiber sources. The cellulose fibers may be supplied in slurry, unsheetedor sheeted form.

[0035] The optimum fiber source utilized in conjunction with thisinvention will depend upon the particular end use contemplated.Generally, pulp fibers made by chemical pulping processes are preferred.Completely bleached, partially bleached and unbleached fibers areapplicable. It may frequently be desirable to utilize bleached pulp forits superior brightness and consumer appeal. For products, such as papertowels, and absorbent pads for diapers, sanitary napkins, catamenials,and other similar absorbent paper products, it is especially preferredto use cellulose fibers derived from southern softwood pulp due to theirpremium absorbency characteristics.

[0036] More preferred cellulose fibers include, but are not limited to,bleached Kraft southern pine fibers sold under the trademark FoleyFluff™ which are available from Buckeye Technologies Inc. of Memphis,Tenn.

[0037] The cellulose fibers may have any fiber length. Typically, longerfibers produce crosslinked cellulose fibers having lower desorptionpressures and water retention values than those produced from shorterfibers.

[0038] Refined and Crosslinked Fibers

[0039] It has been surprisingly and unexpectedly discovered that whencellulose fibers are refined and crosslinked, the resulting fibers havelow median desorption pressures as measured in a capillaryabsorption-desorption cycle and low water retention values (WRVs).Furthermore, these fibers exhibit improved fluid drainage in acquisitionand/or distribution layers compared to similar unrefined fibers.

[0040] The cellulose fibers are crosslinked and have a median desorptionpressure, as determined in a capillary absorption-desorption cycle, of15 cm or less. Without being bound by any theory, the inventors believethis property is the result of intrafiber crosslinking within thecellulose fibers. More desirably, the cellulose fibers of this inventionhave a median desorption pressure, as determined in a capillaryabsorption-desorption cycle, of 14 cm or less; still more desirably, thefibers of this invention have a median desorption pressure, asdetermined in a capillary absorption-desorption cycle of 13 cm or less;and still more desirably, the fibers of the invention here a mediandesorption pressure, as determined in a capillary absorption-desorptioncycle, of 12 cm or less.

[0041] The refined and crosslinked cellulose fibers typically have a WRVof 45 percent or less; more desirably, 38 percent or less; and stillmore desirably 30 percent or less.

[0042] The cellulose fibers may be prepared by refining cellulose fibersto a freeness ranging from about 300 to about 700 ml CSF andcrosslinking the refined fibers. According to one preferred embodiment,the starting cellulose fibers to be refined are wet lap. According toanother preferred embodiment, the cellulose fibers are bleached and/orfluffed prior to being refined. The refined fibers may be crosslinked byany method known in the art, e.g., by reacting the fibers with acrosslinking agent.

[0043] Fibers having improved median desorption pressures and waterretention values can be produced by refining the fibers first and thencrosslinking the fibers with any of a wide variety of crosslinkingagents. Suitable crosslinking agents include, but are not limited to,those described below as well as other polycarboxylic acids, such asaliphatic and alicyclic C₂-C₉ polycarboxylic acids. As used herein, theterm “C₂-C₉ polycarboxylic acid” refers to an organic acid containingtwo or more carboxyl (COOH) groups and from 2 to 9 carbon atoms in thechain or ring to which the carboxyl groups are attached. The carboxylgroups are not included in determining the number of carbon atoms in thechain or ring. For example, 1,2,3-propanetricarboxylic acid would beconsidered to be a C₃ polycarboxylic acid containing three carboxylgroups. Similarly, 1,2,3,4-butanetetracarboxylic acid would beconsidered to be a C₄ polycarboxylic acid containing four carboxylgroups.

[0044] The C₂-C₉ polycarboxylic acids suitable for use as cellulosecrosslinking agents in the present invention preferably includealiphatic and alicyclic acids either olefinically saturated orunsaturated with at least three and preferably more carboxyl groups permolecule if a carbon-carbon double bond is present alpha, beta to one orboth carboxyl groups. Additionally, in order to be reactive inesterifying cellulose hydroxyl groups, a given carboxyl group in analiphatic or alicyclic polycarboxylic acid is preferably separated froma second carboxyl group by no less than two carbon atoms and no morethan three carbon atoms. Without being bound by theory, it appears thatfor a carboxyl group to be reactive, it must be able to form a cyclic 5or 6-member anhydride ring with a neighboring carboxyl group in thepolycarboxylic acid molecule. Where two carboxyl groups are separated bya carbon-carbon double bond or are both connected to the same ring, thetwo carboxyl groups must be in the cis configuration relative to eachother if they are to interact in this manner.

[0045] Novel Crosslinked Fibers

[0046] Another embodiment of the present invention is cellulose fiberscrosslinked with at least one saturated dicarboxylic acid, aromaticdicarboxylic acid, cycloalkyl dicarboxylic acid, bifunctionalmonocarboxylic acid, or amine carboxylic acid which exhibit low mediandesorption pressures as measured in a capillary absorption-desorptioncycle and low water retention values. These crosslinked fibers exhibitimproved fluid drainage in acquisition and/or distribution layers andimproved permeability in absorbent cores.

[0047] Generally, these crosslinked cellulose fibers have a mediandesorption pressure, as determined in a capillary absorption-desorptioncycle, of 25 cm or less. Without being bound by any theory, it isbelieved that this property is the result of intrafiber crosslinkingwithin the cellulose fibers.

[0048] More desirably, the crosslinked cellulose fibers have a mediandesorption pressure, as determined in a capillary absorption-desorptioncycle, of 20 cm or less; still more desirably, the fibers have a mediandesorption pressure, as determined in a capillary absorption-desorptioncycle, of 18 cm or less; still more desirably, the fibers have a mediandesorption pressure, as determined in a capillary absorption-desorptioncycle, of 15 cm or less; still more desirably, the fibers have a mediandesorption pressure, as determined in a capillary absorption-desorptioncycle, of 14 cm or less; still more desirably, the fibers have a mediandesorption pressure, as determined in a capillary absorption-desorptioncycle, of 13 cm or less; and still more desirably, the fibers have amedian desorption pressure, as determined in a capillaryabsorption-desorption cycle, of 12 cm or less.

[0049] The crosslinked cellulose fibers typically have a WRV of 50percent or less; more desirably, 45 percent or less; still moredesirably, 38 percent or less; and still more desirably 30 percent orless.

[0050] The crosslinked cellulose fibers generally have a saturatedcapacity as measured by the procedure described in the examples of thisapplication; Burgeni et al., supra; and Chatterjee et al., supra, of atleast 10 grams of saline per gram of sample (g/g). According to apreferred embodiment, the crosslinked cellulose fibers have a saturatedcapacity of at least 11, 12, 13, 14, or 15 g/g.

[0051] The crosslinked cellulose fibers of the present invention areprepared by crosslinking cellulose fibers with one or more of thecrosslinking agents of the present invention. The median desorptionpressure and water retention value of the crosslinked fibers may bereduced by refining the fibers prior to crosslinking them. Furthermore,the crosslinking reaction may be performed in the presence of one ormore of the crosslinking facilitators of the present invention toimprove the efficacy of the crosslinking agents.

[0052] Refining

[0053] Refining may be performed by any method known in the art,including mechanical refining. Pulp refining involves application ofwork onto fibers, generally but not exclusively carried out in anaqueous slurry. For example, the fibers may be refined by cutting them,thereby reducing the average fiber length. Alternatively, the fibers maybe refined by rubbing the fibers against each other and irregularsurfaces under force or pressure. This causes the exterior fibersurfaces to increase in area, due to scoring and abrasion of thesurfaces. In addition, the work put into the fibers during refiningcauses delamination of the interior fiber walls and surfaces. The resultis weakened fiber walls that allow the fibers to absorb more water andswell to a greater extent than unrefined fibers. Never-dried refinedfibers are also more flexible than similar unrefined never-dried fibers.Furthermore, when consolidated into a sheet, refined fibers, afterdrying, produce greater strength and stiffness in the sheet than isproduced in a sheet of dried unrefined fibers.

[0054] Methods of refining cellulose fibers, including, but not limitedto, beating and fibrillation, are described in J. d'A. Clark, PulpTechnology and Treatment for Paper, 2^(nd) Ed., Chapter 8, pp. 160-183,Chapter 12, pp. 277-305, Chapter 13, pp. 306-355, Chapter 14, pp.356-407, Miller Freeman Pub., San Francisco (1985). A preferred methodof refining cellulose fibers is fibrillation. The cellulose fibers maybe refined with, for example, a disc refiner or a Valley beateravailable from Valley Mill Corporation of Lee, MA. Refining is typicallyperformed at ambient temperature and pressure. For example, cellulosefibers may be refined by running an aqueous slurry of cellulose fibersthrough a Valley beater for 15 minute intervals until the desiredfreeness is obtained.

[0055] Generally, the fibers are wetted or moistened prior to beingrefined. According to one preferred embodiment, the cellulose fibers arebleached prior to being refined.

[0056] The cellulose fibers are broadly refined to a freeness of fromabout 300 to about 700 ml CSF and preferably to a freeness of from about500 to about 700 ml CSF. According to a preferred embodiment, thecellulose fibers are refined to a freeness of from about 650 to about700 CSF. The freeness of cellulose fibers as discussed herein isdetermined by TAPPI Method T-227.

[0057] Crosslinking

[0058] The refined or unrefined cellulose fibers are stiffened byintrafiber covalent crosslinking. Preferably, the cellulose fibers arewet or moist, prior to being reacted with the crosslinking agent andcrosslinking facilitator. Desirably in some embodiments, the cellulosefibers are never-dried cellulose fibers.

[0059] The fibers are crosslinked by reacting them with a crosslinkingagent and, optionally, a crosslinking facilitator of the presentinvention, such as those described below. Preferably, the fibers arecrosslinked while in a highly twisted condition. Typically, the reactionstep is performed under substantially unrestrained conditions, i.e.,individual fibers are free to move without interacting with neighboringfibers and are not under any substantial tension or pressure. The fibersmay be reacted with the crosslinking agent and, optionally, acrosslinking facilitator by curing the fibers in the presence of thecrosslinking agent and, optionally, the crosslinking facilitator.

[0060] Generally, the fibers are crosslinked by (i) mixing them with acrosslinking agent and, optionally, a crosslinking facilitator of thepresent invention and (ii) curing the fibers under conditions sufficientto cause intrafiber crosslinking. An effective amount of crosslinkingagent and optionally crosslinking facilitator to cause formation ofintrafiber crosslink bonds is typically mixed with the cellulose fibers.Preferably, an effective amount of crosslinking facilitator to increasethe number or rate of intrafiber crosslink bonds formed by the reactionof the fibers with the crosslinking agent is mixed with the cellulosefibers. Generally, from about 0.5 to about 40 mole percent andpreferably from about 1 to about 30 mole percent of crosslinking agentand crosslinking facilitator, calculated on a cellulose anhydroglucosemolar basis, is mixed with the fibers. When the crosslinking agent is adicarboxylic crosslinking agent, generally from about 5 to about 21 molepercent of crosslinking agent, calculated on a cellulose anhydroglucosemolar basis, is mixed with the fibers. Generally, from about 1.8 toabout 9 mole percent of crosslinking facilitator, calculated on acellulose anhydroglucose molar basis, is mixed with the fibers. Themixture containing the fibers and crosslinking agent preferably containsfrom about 5 to about 10% by weight of crosslinking agent based upon thedry weight of the fibers.

[0061] After the crosslinking agent is mixed with the fibers, the fibersare preferably separated and individualized, by, for example, fluffingthe fibers or disintegrating and fluffing the fibers. By separating thefibers, intrafiber crosslinking is maximized while interfibercrosslinking is minimized. Preferably, the fibers are crosslinked byformation of intrafiber covalent bonds.

[0062] Cellulose fibers supplied as wet lap, dry lap or other sheetedform may be separated by mechanically disintegrating them to unsheetedform. In the case of dry lap, it is advantageous to moisten the fibers,for example to 40% moisture (60% solids content, based on the totalweight of fiber and water), prior to mechanical disintegration in orderto plasticize the fibers and minimize damage to the fibers.

[0063] When the crosslinking agent is applied to the fibers in anaqueous solution, the fibers are dried prior to being cured. The fibersare preferably dried to remove all the water in the fibers and cured toestablish intrafiber crosslinking. Drying may be performed by any methodknown in the art. Typically, drying is performed by heating the fibersat a temperature of from about 50 to about 225° C. Preferably, drying isperformed at from about 105 to about 175° C. The fibers are typicallydried to constant weight. The temperature of the fibers during thedrying process generally does not exceed 100° C., the boiling point ofwater, irrespective of the temperature at which the fibers are dried,until all of the water has been evaporated from the fibers. As discussedin T. Lindstrom, Paper Structure and Properties, International Fiber andTechnology Series 8, Chapter 5, pp 104-105, Marcel Dekker Inc., New York(1986), drying of cellulose fibers typically leads to an irreversiblereduction in the swelling ability of the fibers on rewetting. Thisphenomenon is commonly referred to as hornification. Without being boundto any theory, it is believed that the microfibrils in the fiber wallsbond together during the drying process thereby reducing the size ofpores in the fiber walls. This results in stiffened fibers, compared tothe fibers before drying. The subsequent curing stage facilitatesformation of the intrafiber covalent bonds that lock in the dried fiberstiffness and geometry.

[0064] Curing is generally performed at a temperature sufficient tocause intrafiber covalent bonds to form. Curing is broadly performed ata temperature of from about 105 to about 225° C. Preferably, thecellulose fibers are cured at a temperature of from about 150 to about190° C. More preferably, they are cured at a temperature of from about160 to about 175° C. Curing may be performed for 15, 30, 45, or 60minutes or longer.

[0065] According to one preferred embodiment, the fibers are crosslinkedby (i) contacting an aqueous solution of the crosslinking agent and,optionally, crosslinking facilitator with an aqueous mixture containingthe cellulose fibers, (ii) removing water from the aqueous mixture,(iii) mechanically separating the fibers into substantially individualform, (iv) drying the fibers, and (v) reacting the fibers with thecrosslinking agent to cause crosslinking in the fibers. Generally, step(ii) involves removing the majority of water from the aqueous mixture.Preferably, enough water is removed from the aqueous mixture to obtain amixture having from about 40 to about 80% by weight of solids, basedupon 100% total weight of fibers and water. According to a morepreferred embodiment, step (ii) involves removing water from the aqueousmixture to obtain a mixture having about 60% by weight of solids, basedupon 100% total weight of fibers and water. The water removal,separating, and drying steps cause the fibers to become highly twisted.The twisted condition generally is at least partially, but less thancompletely, permanently set by the crosslinking reaction.

[0066] For example, the fibers may be crosslinked by the methoddescribed in U.S. Pat. No. 5,190,563, which is hereby incorporated byreference, substituting the crosslinking agents and crosslinkingfacilitators of the present invention for the crosslinking agent in U.S.Pat. No. 5,190,563. In U.S. Pat. No. 5,190,563, cellulosic fibers arecontacted with a solution containing a C₂-C₉ polycarboxylic acidcrosslinking agent. The fibers are then mechanically separated intosubstantially individual form, dried, and reacted with the crosslinkingagent while remaining in substantially individual form so thatintrafiber crosslink bonds form. The individualized cellulosic fibersare contacted with an amount of crosslinking agent effective to causethe fibers to form intrafiber crosslink bonds. Preferably, from about0.5 mole percent to about 6.0 mole percent crosslinking agent,calculated on a cellulose anhydroglucose molar basis, is contacted withthe fibers.

[0067] When the crosslinking agent contains an amino or amine group tobe reacted, the crosslinking agent is preferably activated prior to orsimultaneously with the crosslinking reaction. The term “activated” asused herein refers to modifying the crosslinking agent so that thenitrogen atom of the amino or amine group is in a more reactivecondition, i.e., more prone to reaction. The crosslinking agent may beactivated by any method known in the art. For example, the amine oramino containing crosslinking agent can be reacted with nitrous acid toactivate the nitrogen atom of the amine or amino group.

[0068] The fibers may be crosslinked in the presence a reducing agent(antioxidant) to prevent yellowing of the fibers during the crosslinkingreaction. Suitable reducing agents include, but are not limited to,hypophosphites, such as sodium hypophosphite; sodium bisulfite; sodiumphosphite; and any combination of any of the foregoing. A preferredreducing agent is sodium hypophosphite.

[0069] The fibers may be bleached during or after the crosslinkingreaction to improve their appearance. For example, the fibers may bebleached by reacting them with a bleaching agent. Any bleaching agentknown in the art may be used. Suitable bleaching agents include, but arenot limited to, hydrogen peroxide.

[0070] For example, the bleaching agent may be included in an aqueoussolution containing the crosslinking agent that is applied to thefibers. Preferably, the aqueous solution contains a sufficient amount ofbleaching agent so that the mixture obtained from adding the aqueoussolution to the fibers contains from about 2.5 to about 5% by weight ofbleaching agent, based on the dry weight of the fibers.

[0071] Saturated Dicarboxylic Acid Crosslinking Agents

[0072] The term “saturated dicarboxylic acid” refers to dicarboxylicacids that do not contain any carbon-carbon double or triple bonds. Thesaturated dicarboxylic acids may contain linear or branched aliphaticchains, i.e., they are acyclic. Preferred saturated dicarboxylic acidsinclude, but are not limited to, C₂-C₈ saturated dicarboxylic acids. Theterm “C₂-C₈ saturated dicarboxylic acid” refers to a dicarboxylic acidin which the total number of carbon atoms (including those in thecarboxyl groups) ranges from 2 to 8. Non-limiting examples of C₂-C₈saturated dicarboxylic acids are oxalic acid, malonic acid, succinicacid, glutaric acid, adipic acid, pimelic acid, and suberic acid.Special mention is made of C₂-C₆ saturated dicarboxylic acids and C₂-C₄saturated dicarboxylic acids.

[0073] According to one preferred embodiment, C₃ and higher saturateddicarboxylic acids, such as C₃-C₈ saturated dicarboxylic acids, areapplied to the cellulose fibers in conjunction with a crosslinkingfacilitator, such as oxalic acid.

[0074] Another class of saturated dicarboxylic acids is saturatedhydroxy dicarboxylic acids. The term “saturated hydroxy dicarboxylicacid” refers to saturated dicarboxylic acids that contain at least onehydroxy substituent. Suitable saturated hydroxy dicarboxylic acidsinclude, but are not limited to, C₂-C₈ hydroxy saturated dicarboxylicacids (i.e. those containing from 2 to 8 carbon atoms). Special mentionis made of C₂-C₈ polyhydroxy saturated dicarboxylic acids. Non-limitingexamples of C₂-C₈ hydroxy saturated dicarboxylic acids are tartaricacid, malic acid, saccharic acid, and mucic acid.

[0075] Aromatic Dicarboxylic Acid Crosslinking Agents

[0076] The term “aromatic dicarboxylic acid” refers to aromaticcompounds having the formula HOOC—R—COOH wherein R is a substituted orunsubstituted phenyl group. The term “substituted” as used hereinincludes, but is not limited to, at least one of the followingsubstituents: hydroxy, C₁-C₄ alkoxy, C₁-C₄ alkyl, amino, halogen, andnitro.

[0077] A preferred aromatic dicarboxylic acid has the formula

[0078] where R¹, R², R³, and R⁴ independently are hydrogen, hydroxy,C₁-C₄ alkoxy, C₁-C₄ alkyl, amino, halogen, or nitro. A preferredaromatic dicarboxylic acid is phthalic acid.

[0079] Cycloalkyl Dicarboxylic Acid Crosslinking Agents

[0080] The term “cycloalkyl dicarboxylic acid” refers to cycloalkyldicarboxylic acids that do not contain carbon-carbon double bonds in theα or β positions relative to the carboxyl groups. According to oneembodiment, the cycloalkyl dicarboxylic acid has the formula

[0081] wherein

[0082] R⁶, R⁷, R¹⁰, and R¹¹ are independently hydrogen, hydroxy,halogen, C₁-C₄ alkoxy, C₁-C₄ alkyl, amino, or nitro; and

[0083] R⁸ and R⁹ are independently hydrogen, halogen, C₁-C₄ alkoxy, orC₁-C₄ alkyl. A preferred cycloalkyl dicarboxylic acid is1,2,5,6-tetrahydrophthalic acid.

[0084] Bifunctional Monocarboxylic Acid Crosslinking Agents

[0085] A “bifunctional monocarboxylic acid” refers to an organic acidhaving (a) only one carboxyl group and (b) a functional group, which isnot a carboxyl group, capable of reacting with the carboxyl, carboxylicacid, amino, or hydroxyl groups of a polymer. Preferably, thebifunctional monocarboxylic acid includes only two functional groups,i.e., a carboxyl group and a second functional group.

[0086] Suitable bifunctional monocarboxylic acids include, but are notlimited to, amino acids, salts of haloacetates, hydroxy monocarboxylicacids, and acid derivatives of hydroxy monocarboxylic acids, such asacid esters of hydroxy monocarboxylic acids.

[0087] A preferred salt of a haloacetate is sodium chloroacetate.Without being bound by any theory, it is believed that when an aqueousmixture of sodium chloroacetate and cellulose fibers is dried and cured,an ether is formed by reaction of a cellulose hydroxyl group with thechlorine containing carbon of the sodium chloroacetate molecule. Thisetherification reaction releases a molecule of hydrochloric acid, whichis immediately neutralized by the sodium salt of the newly formedcellulose based acid. At elevated temperature, this acid is availablefor esterification to proximate hydroxyl groups in the fibers, withconcomitant release of water. After the ether and ester formation,sodium chloride remains as a byproduct.

[0088] Suitable hydroxy monocarboxylic acids and acid derivativesthereof include, but are not limited to, glycolic acid, methane sulfonicacid ester of glycolic acid, and para-toluene sulfonic acid ester ofglycolic acid.

[0089] Amine Carboxylic Acid Crosslinking Agents

[0090] Suitable amine carboxylic acids include, but are not limited to,primary, secondary, and tertiary amines and aromatic amines. Preferredprimary amines include, but are not limited to, amino acids. Specialmention is made of amino acids having the formula

H₂N—CH₂—R—C(O)OH

[0091] where R¹² is a bond, C₁-C₁₂ alkyl, or a C₁-C₁₂ alkyl substitutedwith one or more of carboxyl, hydroxy, C₁-C₄ alkoxy, C₁-C₄ alkyl, amino,and nitro.

[0092] Preferred amino acids include, but are not limited to, thosehaving the formula

[0093] where R⁵ is a linear or branched C₁-C₈ alkyl. According to apreferred embodiment, R⁵ is a C₂-C₄ alkyl. Non-limiting examples ofsuitable amino acids include aspartic acid and glutamic acid.

[0094] Other suitable amine carboxylic acid crosslinking agents include,but are not limited to, ethylenedinitrilotetraacetic acid (EDTA).

[0095] Crosslinking Facilitators

[0096] The crosslinking facilitators of the present invention increasethe efficacy of the crosslinking agents. A preferred crosslinkingfacilitator is oxalic acid. Without being bound by any theory, it isbelieved that oxalic acid (pK_(a)=1.23) may serve as an acid catalystfor esterification of the crosslinking agent. Alternatively, oxalic acidmay form a mixed anhydride with the crosslinking agent which thenfacilitates esterification of the cellulose fibers.

[0097] Crosslinking Reversibility

[0098] Fibers crosslinked with the short crosslinking agents of thepresent invention, i.e., those containing 4 carbon atoms or less (e.g. 3carbon atoms or less), such as oxalic acid and sodium chloroacetate, canbe uncrosslinked and crosslinked thereafter. The crosslinking of suchfibers is generally substantially reversible, i.e., at least about 50%by weight of the crosslinked fibers can be uncrosslinked. According toone embodiment, at least about 60, 70, 80, 90, or 95% by weight of thecrosslinked fibers can be uncrosslinked.

[0099] The crosslinked fibers can be uncrosslinked by soaking them inwater for a time sufficient to uncrosslink them. Typically, thecrosslinked fibers are soaked for from about 0.5 to about 4 hours.According to a preferred embodiment, the fibers are soaked for about 2hours. The crosslinked fibers can also be uncrosslinked by subjectingthem to a capillary absorption-desorption cycle as described in theexamples of this application; Burgeni et al., supra; and Chatterjee etal., supra.

[0100] The fibers can be re-crosslinked by drying or drying and curingthem. This phenomenon was not observed with the covalently crosslinkedfibers disclosed in U.S. Pat. Nos. 5,137,537; 5,183,707 and 5,190,563.

[0101] Without being bound by any theory, it is believed that as thecrosslinked fibers absorb water and swell, the crosslinks are strainedas the cellulose polymer chains move apart to accommodate the absorbedwater. When the crosslink molecules are very short in length, as withfibers treated with oxalic acid or sodium chloroacetate (two carbonatoms separate the hydroxyl groups on the neighboring cellulose polymerchains), the strain of the fibers swelling is sufficient to facilitatehydrolysis and cleavage of one of the two covalent bonds that crosslinkthe fibers. In contrast, a citric acid crosslinked fiber as disclosed inU.S. Pat. Nos. 5,137,537; 5,183,707 and 5,190,563 has a much longermolecule bridging the space between cellulose polymer chains (four orfive carbon atoms separate the hydroxyl groups on the neighboringcellulose chains). Consequently the strain on the longer crosslinkingmolecule as the fibers absorb water and swell, is not sufficient tofacilitate cleavage of one of the crosslinking covalent bonds.

[0102] Since the fibers of the present invention can be uncrosslinked,they can be dried and transported or stored in sheeted form instead ofin bulk or baled form. This reduces shipping and storage costs. Thefibers can be re-crosslinked at the destination or whenever desired by,for example, separating and curing them. Once the fibers arere-crosslinked, they can, for example, be incorporated into an absorbentstructure.

[0103] The terms “reversible crosslinked fibers” and “reversiblecrosslinked cellulose fibers” as used herein refer to crosslinked fibersor crosslinked cellulose fibers in which at least about 50, 60, 70, 80,90, or 95% by weight of the crosslinked fibers become uncrosslinkedafter being soaked in water for up to 4 hours and in which at least 50,60, 70, 80, 90, or 95% of the uncrosslinked fibers can re-crosslinked bydrying the fibers at a temperature of 105° C. or higher.

[0104] The reversible crosslinked fibers of the present invention areparticularly suitable for use in an absorbent core containingsuperabsorbent polymer (SAP) particles. The crosslinked fibers separatethe SAP particles and provide channels for fluid flow around the SAPparticles from the wet to the dry areas of the absorbent core.Additionally, the reversible crosslinked fibers facilitate absorption oflarge volumes of urine (or other fluid) over a short period of time(e.g., a gush). Once urine or other fluid enters the absorbent core, thecrosslinked fibers begin to uncrosslink. According to one embodiment,after 0.5 to 4.0 hours of exposure to the urine or other fluid, themajority of wet fibers are uncrosslinked. The uncrosslinked fibers holdand retain the urine or other fluid to a greater extent than fibers thatare permanently crosslinked. As a result, the absorbent core hasimproved initial gush capacity compared to absorbent cores containingconventional fluff fibers and improved rewet performance compared toabsorbent cores containing permanently crosslinked fibers.

[0105] Absorbent Structures

[0106] The cellulose fibers of the present invention may be incorporatedinto any disposable or non-disposable absorbent structure intended toabsorb and contain body exudates, and which are generally placed orretained in proximity with the body of the wearer. Such absorbentstructures are commonly employed in disposable and non-disposableabsorbent articles. Examples of disposable absorbent articles include,but are not limited to, infant diapers, adult incontinence products,training pants, sanitary napkins and other feminine hygiene products.Examples of absorbent structures in which the cellulose fibers of thepresent invention may be incorporated include, but are not limited to,those described in International Publication Nos. WO 98/47456, WO99/63906, WO 99/63922, WO 99/63923, WO 99/63925, WO 00/20095, WO00/38607, WO 00/41882, WO 00/71790, and WO 00/74620 and U.S. Pat. No.5,695,486, all of which are hereby incorporated by reference.

[0107] Acquisition and Distribution Layers

[0108] The cellulose fibers of the present invention may be incorporatedinto an acquisition, distribution, or acquisition-distribution layer.Such layers are commonly employed in absorbent structures contained indisposable absorbent articles. The acquisition and/or distribution layermay be prepared and incorporated into an absorbent structure by anymethod known in the art. According to one embodiment, the absorbentstructure comprises a top layer, which includes the acquisition and/ordistribution layer of the present invention, and a bottom storage layer(also known as an absorbent core). The acquisition and distributionlayer may be a single layer or two separate layers, i.e., a topacquisition layer and a lower distribution layer. The lower distributionlayer rapidly drains fluid from the acquisition layer and distributesthe fluid into the storage layer.

[0109] The acquisition layer of the present invention typically includesfrom about 90 to about 100% by weight of the cellulose fibers of thepresent invention, based upon 100% total weight of acquisition layer.The density of the acquisition layer broadly ranges from about 0.04 toabout 0.07 g/cm³.

[0110] Absorbent Core

[0111] The cellulose fibers of the present invention may be incorporatedinto an absorbent core (also known as a storage layer). The absorbentcore may include any material known in the art that absorbs liquid.Suitable materials include, but are not limited to, fibrous batts orwebs constructed of defiberized, loose, fluffed, and/or hydrophiliccellulosic fibers or the fibers of the present invention; superabsorbentpolymer (SAP) particles, granules, flakes or fibers (collectivelyparticles); and any combination of the foregoing. Generally, SAPparticles are capable of absorbing many times their weight of liquid andsignificantly increase the absorbent capacity of the absorbent corewithout substantially increasing the bulkiness of the layer.

[0112] The term superabsorbent polymer particle or SAP particle isintended to include any particulate form of superabsorbent polymer,including irregular granules, spherical particles (beads), powder,flakes, staple fibers and other elongated particles. SAP refers to anormally water-soluble polymer which has been cross-linked to render itsubstantially water insoluble, but generally capable of absorbing atleast ten, and preferably at least fifteen, times its weight of aphysiological saline solution. The SAP particles may be of any size orshape. Numerous examples of superabsorbers and their methods ofpreparation may be found, for example, in U.S. Pat. Nos. 4,102,340;4,467,012; 4,950,264; 5,147,343; 5,328,935; 5,338,766; 5,372,766;5,849,816; 5,859,077; and Re. 32,649. Examples of suitable SAP particlesinclude, but are not limited to, starch graft copolymers, such ashydrolyzed starch-acrylate graft co-polymer; cross-linkedcarboxymethylcellulose and derivatives thereof; and modified hydrophilicpolyacrylates, such as saponified acrylic acid ester-vinyl co-polymer,neutralized crosslinked polyacrylic acid, and cross-linked polyacrylatesalts.

[0113] Preferably, the SAP particles form hydrogels upon absorbingfluid. More preferably, the SAP particles have high gel volumes or highgel strength as measured by the shear modulus of the hydrogel. Such SAPparticles typically contain relatively low levels of polymeric materialsthat can be extracted by contact with synthetic urine (so-calledextractables). An example of such SAP particles is starch graftpolyacrylate hydrogel, available as IM1000® from Hoechst-Celanese ofPortsmouth, Va. Other examples of hydrogels containing SAP particlesinclude, but are not limited to, those sold under the trademarksSANWET™, available from Sanyo Kasei Kogyo Kabushiki of Japan; SUMIKAGEL™ available from Sumitomo Kagaku Kabushiki Haishi of Japan; andFAVOR™ available from Stockhausen of Garyville, La.; and ASAP™ availablefrom BASF of Aberdeen, Miss.

[0114] According to one preferred embodiment, the absorbent corecontains (a) SAP particles and (b) fluff fibers, matrix fibers, thefibers of the present invention, or any combination of the foregoing.The fibers provide structural integrity and avenues for the passage offluid through the absorbent core.

[0115] According to another embodiment, the absorbent core contains fromabout 30 to about 70% by weight of SAP particles and from about 70 toabout 30% by weight of the cellulose fibers of the present invention,based on 100% total weight of the absorbent core. Generally, the weightdensity of the absorbent core ranges from about 0.15 to about 0.25g/cm³.

[0116] According to yet another embodiment, the absorbent core comprisesSAP particles and reversible crosslinked fibers of the presentinvention. According to one preferred embodiment, the reversiblecrosslinked fibers are crosslinked with oxalic acid, sodiumchloroacetate, or a mixture thereof. Generally, the absorbent corecontains from about 30 to about 70% by weight of SAP particles and fromabout 70 to about 30% by weight of reversible crosslinked fibers of thepresent invention, based on 100% total weight of the absorbent core.According to another preferred embodiment, the acquisition and/ordistribution layers and the absorbent core contain fibers crosslinkedwith oxalic acid.

[0117] The absorbent structure of the present invention may beincorporated into disposable and non-disposable absorbent articles, suchas diapers, adult incontinence pads and feminine hygiene articles. Theabsorbent article can include a liquid pervious top-sheet above theacquisition and/or distribution layer, whose function is to allow thepassage of fluid to the acquisition and/or distribution layer, and aliquid impervious back-sheet, whose function is to contain the fluid andto prevent it from passing through the absorbent article to the garmentof the wearer of the absorbent article.

[0118] According to one embodiment, the absorbent structure of thepresent invention is incorporated into a disposable infant diaper whichgenerally includes a front waistband area, a rear waistband area, and acrotch region there between. The structure of the diaper generallyincludes a liquid pervious top-sheet, a liquid impervious back-sheet,the absorbent structure, elastic members, and securing tabs.Representative disposable diaper designs may be found, for example, inU.S. Pat. Nos. 4,935,022 and 5,149,335.

[0119] According to another embodiment, the absorbent structure of thepresent invention is incorporated into a feminine hygiene pad, such asthat disclosed in U.S. Pat. No. 5,961,505.

[0120] The following examples illustrate the invention withoutlimitation. All parts and percentages are given by weight unlessotherwise indicated. All the crosslinking agents, crosslinkingfacilitators, and other chemical reagents used in the examples areavailable from Aldrich Chemical Company of Milwaukee, Wis.

[0121] Measurement of Capillary and Desorption Pressures and SaturatedCapacity

[0122] The capillary absorption and desorption pressures were determinedaccording to the procedure described in “Capillary Sorption Equilibriain Fiber Masses”, A. A. Burgeni and C. Kapur, Textile Research Journal,37:356-366 (1967). This procedure is described in detail below.

[0123] A 0.75 g sample of individualized fibers is formed into a discapproximately 55-60 mm in diameter. The sample is placed on the frit ofa 150 ml coarse frit Pyrex® glass funnel, Corning No. 36060 (availablefrom VWR of Suwanee, Ga.). A weight sufficient to apply 0.22 psipressure, of diameter comparable to that of the sample is placed on thesample. The bottom of the funnel is attached to an adapter withdecreasing diameter such that Tygon® tubing No. R-3603, approximatelytwo feet in length, is attached to the adapter on one end, and on theother end to a fluid reservoir resting on an electronic scale capable ofmeasuring 0.01 g. The tubing is attached to the side of the reservoir atthe bottom. The fluid reservoir contains 0.9% saline solution. Theheight of the saline solution in the fluid reservoir is approximately 1inch above the tubing attachment. The tubing is filled with saline, asis the funnel below the frit, such that the frit is damp with thesaline, but no saline is above the frit. The saline column from thereservoir to the frit is continuous without any air in the column.

[0124] The absorption cycle is as follows. Starting at a height of, forexample, 20, 30, or 80 cm above the level of saline in the reservoir,the sample is allowed to absorb saline to equilibrium or steady state.Steady state is determined as no change in the weight of saline on theelectronic scale beneath the reservoir greater than 0.04 g over a periodof one minute. When steady state is achieved, the sample is lowered 5 cmcloser to the level of saline in the reservoir and held there untilequilibrium is achieved. The sample is lowered another 5 cm and theprocedure repeated. When the sample is at equilibrium at the same heightas the saline level in the reservoir, the sample is subjected to adesorption cycle by reversing the procedure, ie., by moving the sampleupward in 5 cm increments.

[0125] The weight of saline in the sample at the same level as thesaline in the reservoir is the saturated capacity of the sample. Theheight (reported in cm) of the sample above the saline level in thereservoir at 50% of saturated capacity on the downward (absorption)cycle (median absorption pressure) and on the upward (desorption) curve(median desorption pressure) are determined by interpolation. The valueof saturated capacity is reported in table as grams of saline per gramof sample.

EXAMPLES 1-8 Examples 1-8 in Table 1 were Prepared as Follows

[0126] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc. ofMemphis, Tenn., were slurried in water and refined in a valley beater atambient temperature and pressure to the appropriate degree of freeness.The fibers were centrifuged, separated by hand, and air dried to 60%solids. The fibers were crosslinked with the appropriate concentrationof citric acid (dry fiber basis) by spraying the fibers with an aqueoussolution of citric acid having sufficient dilution to bring the sheet to40% solids content. The fibers were then air dried to 60% solidscontent, fluffed, and dried to constant weight and heated for anadditional 30 minutes at the temperature indicated in Table 1.

[0127] The water retention value, saturation capacity, capillaryabsorption pressure, and capillary desorption pressure of the curedcellulose fibers were determined. The water retention value of the curedcellulose fibers was determined according to the procedure described inTAPPI Useful Methods, UM 256. The capillary absorption and desorptionpressures were determined according to the procedure described above.

[0128] The results are shown in Table 1.

COMPARATIVE EXAMPLE 9

[0129] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc. ofMemphis, Tenn., were dried at 100° C. The water retention value,saturation capacity, capillary absorption pressure, and capillarydesorption pressure of the cured cellulose fibers were determined asdescribed in Example 1.

[0130] The results are shown in Table 1 below.

COMPARATIVE EXAMPLE 10

[0131] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc., weresprayed with sufficient water to produce a 40% solids content. Thefibers were air dried to 60% solids content, mechanically fluffed, anddried to constant weight at 150° C. The fibers were then heated for anadditional 30 minutes at the same temperature. The water retentionvalue, saturation capacity, capillary absorption pressure, and capillarydesorption pressure of the cured cellulose fibers were determined asdescribed in Example 1.

[0132] The results are shown in Table 1.

COMPARATIVE EXAMPLE 11

[0133] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc., wereslurried in water and refined in a valley beater to a freeness ofapproximately 500 ml CSF. The fibers were centrifuged, separated byhand, air dried to 60% solids content, fluffed, and dried to constantweight at 150° C. The fibers were then heated for an additional 30minutes at 150° C. The water retention value, saturation capacity,capillary absorption pressure, and capillary desorption pressure of thecured cellulose fibers were determined as described in Example 1.

[0134] The results are shown in Table 1.

COMPARATIVE EXAMPLE 12

[0135] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc., wereslurried in water and refined in a valley beater to a freeness ofapproximately 500 ml CSF. The fibers were centrifuged, separated byhand, and air dried to 60% solids. The cellulose fibers were sprayedwith an aqueous sulfuric acid solution at pH 3 to 40% solids content inorder to adjust the pH of the fiber water mixture to the same pH as thatobserved in Example 1, when the fibers were treated with aqueous citricacid. The fibers were air dried to 60% solids content, fluffed, anddried to constant weight at 150° C. The fibers were then heated for anadditional 30 minutes at 150° C. The water retention value, saturationcapacity, capillary absorption pressure, and capillary desorptionpressure of the cured cellulose fibers were determined as described inExample 1.

[0136] The results are shown in Table 1.

COMPARATIVE EXAMPLE 13

[0137] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc., werecrosslinked with 5% citric acid (dry fiber basis) by spraying the fiberswith an aqueous solution of citric acid having sufficient dilution tobring the sheet to 40% solids content. The sheet was air dried to 60%solids content, mechanically fluffed, dried to constant weight at 150°C. The sheet was then heated for an additional 30 minutes at 150° C. Thewater retention value, saturation capacity, capillary absorptionpressure, and capillary desorption pressure of the cured cellulosefibers were determined as described in Example 1.

[0138] The results are shown in Table 1.

COMPARATIVE EXAMPLE 14

[0139] The procedure described in Comparative Example 13 was repeated,except the fibers were crosslinked with 10% citric acid instead of 5%citric acid. The results are shown in Table 1. TABLE 1 CrosslinkedAbsorption Desorption (% Citric Acid) Cure Saturation Pressure at 50%Pressure at 50% Freeness (Dry Fiber Temperature WRV Capacity ofSaturated of Saturated Example (ml CSF) Basis) (° C.) (%) (g/g) Capacity(cm) Capacity (cm) Example 1 500 5 150 44.8 8.2 3.9 13.4 Example 2 50010 150 38.1 8.7 4.4 13.5 Example 3 300 5 150 42.7 8.2 5.3 14.7 Example 4300 10 150 41.5 8.6 5.1 14.9 Example 5 500 5 175 36.7 8.5 4.4 14.0Example 6 500 10 175 29.5 8.3 3.1 11.9 Example 7 300 5 175 33.4 7.9 4.215.0 Example 8 300 10 175 29.9 7.5 3.0 13.0 Comparative Example  9 740 —100 83.0 9.3 12.3 30.5 Comparative Example 10 740 — 150 73.6 11.0 12.027.4 Comparative Example 11 500 — 150 92.1 8.2 8.1 20.7 ComparativeExample 12 500 — 150 78.6 8.3 5.3 16.3 Comparative Example 13 740 5 15043.3 12.3 — 17.9 Comparative Example 14 740 10 150 38.4 12.1 6.7 18.4

[0140] As shown by the results in Table 1, the refined and crosslinkedfibers exhibited lower WRVs and desorption pressures than similarunrefined and crosslinked fibers.

EXAMPLE 15

[0141] Refining

[0142] An aqueous slurry containing 2.75-3.25% by weight of never driedFoley Fluff™ fibers, available from Buckeye Technologies Inc., wasprepared. The aqueous slurry was passed through a Bauer Model No. 444,24″ pump through refiner, at ambient temperature and pressure. The Bauerrefiner plates were No. A24313.

[0143] The refiner was operated at a current of 178 amps and a slurryflow rate of 255 gallons per minute. These conditions produced loadingof 30-60 horsepower hours per bone dry short ton of fibers. The fibersproduced had a freeness of 680 ml CSF.

[0144] Demineralizing

[0145] The refined pulp slurry was pumped to a false bottom tank at aconsistency of 2.75-3.25%. While stirring the pulp slurry, sulfuric acidwas added until a nominal pH of 2.0 was obtained. After at least 10minutes of stirring, the aqueous slurry was allowed to de-water throughthe false bottom screen for a minimum of 3 hours. The slurry was thendiluted with sodium softened water to 2.0% consistency and the pH wasadjusted to 4.5-5.0.

[0146] Sheeting

[0147] Pulp sheeting was performed on a paper machine available fromSandy Hill Corporation of Hudson Falls, N.Y. The deckle (sheet width)was a maximum of 36 inches. The 2% pulp slurry was pumped through astuff box and a basis weight valve and into a white water silo at acontrolled flow rate. The temperature in the silo was increased to130-150° F. with direct steam and the slurry in the silo was dilutedwith white water to a consistency of 1.0-1.25%.

[0148] The slurry was then fed into a paper machine headbox and movedonto a moving wire at the Fordrinier section of the paper machine.Natural drainage and vacuum assisted drainage were provided until theformed sheet exited the couch press at about 32% consistency. Afterformation of the sheet, but before the couch press, the wet sheet wastrimmed with two jets of water to deckle of 24 inches. The sheet offibers having a consistency of about 32% then passed through two wetpresses where further water removal and sheet densification occurred.After exiting the second wet press, the sheet of fibers entered a firstdryer section at approximately 48% consistency. In the first dryersection, the pulp sheet passed over thirteen rotating steam cans atapproximately 300-325° F. The pulp sheet was then passed over eightrotating steam cans in a second dryer section and exited the dryer at amoisture content of 4-8%. The sheet was wound into a roll at a deckle ofabout 23 inches. The basis weight of this rolled sheet was approximately0.126 pound per square foot and the density was approximately 0.60 gramsper cubic centimeter.

[0149] Slitting

[0150] The rolls of pulp were rewound onto a new core and slit intosmaller rolls, each 10 inches wide.

[0151] Chemical Application

[0152] A 10 inch wide roll of the pulp sheet was unwound and slowlypassed through a puddle press. At the nip of the puddle press was anaqueous solution of citric acid and sodium hypophosphite. The sodiumhypophosphite moderates pulp darkening at high temperature. The weightconcentrations of citric acid and sodium hypophosphite in the floodednip were approximately 14% and 7%, respectively. Through the puddlepress, the sheet absorbed enough of the aqueous solution to reach amoisture content of about 40%.

[0153] Sheet Disintegration and Fluffing

[0154] Following the puddle press, the sheet was picked apart intosmaller pieces through a shredder, a pre-breaker and a picker. Thedisintegrated pulp was then blown into the inlet of a Sunds DefibratorModel 3784 RO Fluffer, available from Sunds Defibrator, AB of Sundsvall,Sweden, with a gap setting of 5.5 mm. The defibrator fluffed the pulpinto masses of separated fibers. The fluffed pulp was swept out of theRO Fluffer with a high velocity stream of hot air at approximately 380°F.

[0155] Drying and Curing

[0156] The hot air flow that conveyed the fluffed fiber out of the ROFluffer was boosted with a fan through a flash dryer where all or almostall of the water in the fibers was evaporated. The dried pulp fell ontoa mechanical inlet conveyor forming a low density high bulk bed on theconveyor. The fibers were then transported into a Proctor & SchwartzK16476 tunnel dryer, available from Proctor & Schwartz, Inc. of Horsham,Pa. Through a series of hot circulating air flows, the fluffed fiber bedwas heated and then allowed to cool through three chambers in the dryer.In chamber 1, the bed temperature reached 325-330° F. In chamber 2, thebed temperature increased to 385-390° F. In chamber 3, the bedtemperature decreased to 355-360° F. The total time in the tunnel dryerwas approximately 11.5 minutes.

[0157] Baling

[0158] The crosslinked fibers fell off the conveyer from the exit sideof the tunnel dryer into a baler model no. 3445, available from AmericanBaler Company of Bellevue, Ohio, where the material was compressed intobales weighing approximately 70-80 pounds.

EXAMPLE 16

[0159] The samples in Table 2 were prepared as follows.

[0160] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc. ofMemphis, Tenn., were slurried in water and refined in a Bauer Model No.444, 24″ pump through refiner, at ambient temperature and pressure tothe appropriate degree of freeness. Optionally, the fibers were washedwith dilute sulfuric acid (acid wash) to remove minerals. The refinedfibers were sheeted and dried.

[0161] A piece of the sheet was submerged in a tray containing asolution of the crosslinking agent and oxalic acid. The piece was thenflipped over and submerged in a second solution of the crosslinkingagent and oxalic acid. The combined solutions contained 10% of thecrosslinking agent (dry fiber basis) and 5% oxalic acid (dry fiberbasis). The solutions had sufficient dilution to bring the piece to 40%solids content. The piece was placed in a sealed polyethylene bag for 1hour. The fibers were air dried to 60% solids content, fluffed, anddried to constant weight and heated for an additional 30 minutes at 175°C.

[0162] The water retention value, saturation capacity, capillaryabsorption pressure, and capillary desorption pressure of the curedcellulose fibers were determined. The water retention value of the curedcellulose fibers was determined according to the procedure described inTAPPI Useful Methods, UM 256. The capillary absorption and desorptionpressures were determined according to the procedure described above.

[0163] The results are shown in Table 2. TABLE 2 Absorption DesorptionSaturated Pressure (cm) @ Pressure (cm) @ Crosslinking Freeness Acid WRVCapacity 50% of Saturated 50% of Saturated Carboxyl Sample Agent (mlCSF) Washed (%) (g/g) Capacity Capacity (meq/kgm) a Succinic Acid 740 No31.5 13.9 6.8 14.7 343.1 b Succinic Acid 570 No 36.3 13.5 7.2 17.3 294.1c Succinic Acid 570 Yes 36.0 13.7 7.2 16.1 303.9 d Adipic Acid 740 No36.0 13.5 7.4 16.4 228.6 e Adipic Acid 570 No 39.1 12.6 7.0 16.4 190.6 fAdipic Acid 570 Yes 40.0 13.1 7.8 18.3 179.3 g Pimelic Acid 740 No 35.414.7 6.4 17.5 231.4 h Pimelic Acid 570 No 39.4 13.9 6.9 17.7 209.0 iPimelic Acid 570 Yes 37.9 12.2 7.3 17.3 200.0 j Malonic Acid 740 No 32.816.1 7.2 18.9 75.6 k Malonic Acid 570 No 36.5 11.6 6.1 16.8 78.2 lMalonic Acid 570 Yes 37.7 12.7 7.1 15.8 85.5

EXAMPLE 17

[0164] The samples in Table 3 were prepared as follows.

[0165] Unrefined cellulose fibers having a freeness of 740 ml CSF,available as Foley Fluff™ fibers from Buckeye Technologies Inc. ofMemphis, Tenn., were slurried in water and refined in a Bauer Model No.444, 24″ pump through refiner, at ambient temperature and pressure tothe appropriate degree of freeness. The refined fibers were sheeted anddried. If the fibers were wet lap, they were then centrifuged.

[0166] A piece of the sheet was submerged in a tray containing asolution of the crosslinking agent, sodium hypophosphite, and,optionally, oxalic acid. The piece was then flipped over and submergedin a second solution of the crosslinking agent, sodium hypophosphite,and, optionally, oxalic acid. The combined solutions contained 10% ofthe crosslinking agent (dry fiber basis), 5% sodium hypophosphite (dryfiber basis) and, optionally, 1% oxalic acid (dry fiber basis). Thesolutions had sufficient dilution to bring the piece to 40% solidscontent. The piece was placed in a sealed polyethylene bag for 1 hour.The fibers were air dried to 60% solids content, fluffed, and dried toconstant weight and heated for an additional 30 minutes at 175° C.

[0167] The water retention value, saturation capacity, capillaryabsorption pressure, and capillary desorption pressure of the curedcellulose fibers were determined. The water retention value of the curedcellulose fibers was determined according to the procedure described inTAPPI Useful Methods, UM 256. The capillary absorption and desorptionpressures were determined according to the procedure described above.

[0168] The results are shown in Table 3. TABLE 3 Absorption DesorptionPressure (cm) Pressure (cm) Cross- Concentration Saturated @ 50% of @50% of linking of Oxalic Acid Freeness WRV Capactiy Saturated SaturatedCarboxyl Sample Agent (% w/w) (ml CSF) (%) (g/g) Capacity Capacity(meq/kgm) a Oxalic Acid — 740 48.3 12.0 6.2 18.9 292.5 b Maleic Acid —740 33.8 15.3 4.7 16.4 529.5 c Succinic Acid — 740 38.2 14.0 5.3 16.7414.8 d Adipic Acid — 740 45.7 13.4 5.7 17.2 343.9 e Succinic Acid 1 74037.8 14.5 4.6 14.9 432.2 f Adipic Acid 1 740 45.3 13.0 6.1 17.8 356.3 gCitric Acid — 740 33.1 15.6 4.4 15.2 442.0 h Oxalic Acid — 680 47.3 10.56.5 18.4 287.7 i Maleic Acid — 680 34.2 14.4 4.2 15.0 530.2 j SuccinicAcid — 680 38.0 12.8 5.8 16.6 436.8 k Adipic Acid — 680 49.6 11.8 5.415.6 351.3 l Succinic Acid 1 680 37.0 14.0 4.4 14.3 424.6 m Adipic Acid1 680 45.8 12.3 6.3 17.7 358.5 n Citric Acid — 680 31.3 15.8 3.7 13.5450.3 o Oxalic Acid — 570 59.9 3.2 5.2 12.7 432.0 (Wet Lap) p MaleicAcid — 570 41.6 5.1 3.8 7.7 474.0 (Wet Lap) q Succinic Acid — 570 34.88.8 3.2 8.3 466.1 (Wet Lap) r Adipic Acid — 570 50.1 9.0 3.4 8.7 487.5(Wet Lap) s Succinic Acid 1 570 36.1 8.8 3.3 8.7 415.4 (Wet Lap) tAdipic Acid 1 570 44.2 8.6 3.3 8.9 422.1 (Wet Lap) u Citric Acid — 57035.0 9.7 3.3 9.2 468.6 (Wet Lap) v Oxalic Acid — 570 49.6 11.7 5.3 15.8405.8 w Maleic Acid — 570 41.5 12.4 5.9 16.7 450.7 x Succinic Acid — 57037.2 13.4 4.8 15.1 510.1 y Adipic Acid — 570 44.9 11.3 5.3 16.0 328.0 zSuccinic Acid 1 570 40.7 12.0 5.4 16.2 455.5 aa Adipic Acid 1 570 48.611.4 5.9 16.7 371.0 bb Citric Acid — 570 39.9 13.7 3.8 12.9 481.1

EXAMPLE 18

[0169] The samples described in Table 4 below were prepared as follows.

[0170] Stock solutions of the crosslinking agents and crosslinkingfacilitators identified in Table 4 were prepared by dissolving theindicated amounts of crosslinking agent and crosslinking facilitator in22.5 g of distilled water.

[0171] Samples were treated with the appropriate solution as follows. A15 g (dry basis) sample of sheeted, unrefined cellulose fibers having afreeness of 740 ml CSF, available as Foley Fluff™ fibers from BuckeyeTechnologies Inc. of Memphis, Tenn., was treated with the appropriatestock solution containing the crosslinking agent and crosslinkingfacilitator. This reduced the fiber solids content of the mixture to40%. The mixture was placed in a sealed container for 60 minutes atambient temperature and then air dried to 60% solids content. The fiberswere mechanically separated, individualized, and fluffed in a laboratoryfluffer and dried to constant weight in a forced air oven at 175° C. Thefibers were cured for 30 minutes at the same temperature.

[0172] A control was prepared as follows. A dry unrefined sheet ofcellulose fibers having a freeness of 740 ml CSF, available as FoleyFluff™ fibers from Buckeye Technologies Inc. of Memphis, Tenn., wasdiluted to a 40% solids content with water. The pH of the mixture wasadjusted to 3 with sulfuric acid. The stock solutions applied to thefibers in the procedure above typically have a pH of 3. The mixture wasthen placed in a sealed container for 60 minutes at ambient temperatureand then air dried to a 60% solids content. The fibers were mechanicallyseparated, individualized and fluffed in a laboratory fluffer and driedto constant weight in a forced air oven at 175 ° C. The fibers wereheated for an additional 30 minutes at the same temperature.

[0173] Fibers from an acquisition-distribution layer of a Pampers®disposable diaper, available from Proctor and Gamble of Cincinnati,Ohio, were obtained and used as a second control.

[0174] The water retention value, saturation capacity, capillaryabsorption pressure, and capillary desorption pressure of the fiberswere determined. The water retention value of the cellulose fibers wasdetermined according to the procedure described in TAPPI Useful Methods,UM 256. The capillary absorption and desorption pressures and saturatedcapacity were determined by the procedure described above.

[0175] The results are shown in Table 4. TABLE 4 Amount Amount of Cross-Absorption Desorption of Cross- linking Pressure (cm) Pressure (cm)Cross- linking Agent Cross- Facilitator Saturated @ 50% of @ 50% oflinking in Stock linking in Stock WRV Capactiy Saturated SaturatedSample Agent Solution (g) Facilitator Solution (g) (%) (g/g) CapacityCapacity a None (Control) — None — 59.4 13.6 10.7 24.6 b Pampers ® — — —44.9 6.7 4.4 18.2 (Control) c Sodium 0.75 None — 49.8 12.7 9.5 22.2Chloroacetate d Sodium 1.5 None — 44.6 11.1 8.2 18.5 Chloroacetate eOxalic Acid 0.15 None — 47.2 12.9 9.1 23.6 f Oxalic Acid 0.75 None —42.9 13.6 8.6 21.5 g Oxalic Acid 1.5 None — 38.5 15.3 7.6 17.8 hSuccinic Acid 1.5 None — 39.8 14.2 7.5 20.4 i Succinic Acid 1.5 OxalicAcid 0.15 33.8 14.3 7.7 16.6 j Succinic Acid 1.5 Oxalic Acid 0.75 31.513.9 6.8 14.7 k Adipic Acid 1.5 None — 50.5 14.1 8.0 22.3 l Adipic Acid1.5 Oxalic Acid 0.15 38.9 13.7 8.6 19.8 m Adipic Acid 1.5 Oxalic Acid0.75 36.0 13.5 7.4 16.4 n Malonic Acid 1.5 None — 39.5 13.5 7.8 22.8 oMalonic Acid 1.5 Oxalic Acid 0.75 32.8 16.1 7.2 18.9 p Glutaric Acid 1.5None — 37.1 15.2 7.5 23.4 q Glutaric Acid 1.5 Oxalic Acid 0.75 33.0 15.97.3 21.9 r Pimelic Acid 1.5 None — 42.7 14.8 7.4 23.1 s Pimelic Acid 1.5Oxalic Acid 0.75 35.4 14.7 6.4 17.5 t Suberic Acid 1.5 None — 56.0 13.78.3 25.7 u Suberic Acid 1.5 Oxalic Acid 0.75 41.0 14.2 7.2 21.4 vPhthalic Acid 1.5 None — 55.4 13.9 8.4 26.3 w Phthalic Acid 1.5 OxalicAcid 0.75 42.5 13.0 9.0 24.6 x Tetrahydro- 1.5 None — 53.4 12.6 8.4 22.5phthalic Acid y Tetrahydro- 1.5 Oxalic Acid 0.75 39.4 13.3 7.4 19.9phthalic Acid z Fumaric Acid 1.5 None — 44.5 13.9 9.1 24.1 aa FumaricAcid 1.5 Oxalic Acid 0.75 41.5 12.7 8.4 19.2 bb Glycolic Acid 1.5 None —47.8 13.3 8.7 22.0 cc Glycolic Acid 1.5 Oxalic Acid 0.75 39.6 14.0 7.317.4 dd Tartaric Acid 1.5 None — 34.3 13.7 7.3 18.5 ee Tartaric Acid 1.5Oxalic Acid 0.75 32.2 13.1 7.3 18.3 ff Malic Acid 1.5 None — 31.6 14.56.9 19.0 gg Malic Acid 1.5 Oxalic Acid 0.75 30.1 14.0 6.6 18.7 hhSaccharic Acid 1.5 None — 49.6 13.4 9.4 22.8 ii Saccharic Acid 1.5Oxalic Acid 0.75 41.1 11.4 8.2 19.4 jj Mucic Acid 1.5 None — 55.9 12.49.9 19.0 kk Mucic Acid 1.5 Oxalic Acid 0.75 40.3 11.9 8.4 17.1 llAspartic Acid 1.5 None — 55.2 13.9 9.5 26.6 mm Aspartic Acid 1.5 OxalicAcid 0.75 37.5 14.6 6.5 16.5 nn Glutamic Acid 1.5 None — 52.8 13.8 7.825.1 oo Glutamic Acid 1.5 Oxalic Acid 0.75 37.4 14.2 7.0 17.3 pp EDTA1.5 None — 50.7 12.1 8.6 21.9 qq EDTA 1.5 Oxalic Acid 0.75 39.7 12.1 8.018.4

EXAMPLE 19

[0176] A sample of crosslinked fibers was prepared by the methoddescribed in Example 18 with 1.5 g sodium chloroacetate. A second samplewas prepared with 1.5 g oxalic acid instead of sodium chloroacetate.

[0177] For comparison purposes, a sample of crosslinked fibers wasprepared by the procedure in Example 18 using a stock solutioncontaining 10% by weight of citric acid.

[0178] The samples were subjected to a first capillaryabsorption-desorption cycle by the procedure described above. Thesamples were then subjected to a second capillary absorption-desorptioncycle by the same procedure. The observed absorption and desorptionpressures for both cycles are shown in Table 5 below.

[0179] This test was repeated with uncrosslinked Foley Fluff™ fibers.TABLE 5 Crosslinking Agent Oxalic Sodium Citric None Acid ChloroacetateAcid First Cycle Saturated Capacity (g/g) 13.6 15.3 12.2 18.5 AbsorptionPressure (cm) @ 10.7 7.6 10.5  7.0 50% of Saturated Capacity DesorptionPressure (cm) @ 24.6 17.8 18.2 13.5 50% of Saturated Capacity SecondCycle Saturated Capacity (g/g) 11.8 13.1 9.6 16.5 Absorption Pressure(cm) @ >30 >30 >30 15.3 50% of Saturated Capacity Desorption Pressure(cm) @ 25.8 29.3 24.7 18.4 50% of Saturated Capacity

EXAMPLE 20

[0180] Samples as described in Example 19 were prepared and subjected totwo capillary absorption-desorption cycles. The samples were driedovernight in a forced air oven at 105° C. Alternatively, the sampleswere dried to constant weight at 105° C. and heated for an additional 30minutes (cured) at 175° C. The saturated capacity and absorption anddesorption pressures of the samples was determined.

[0181] The results are shown in Table 6. TABLE 6 Crosslinking AgentOxalic Sodium Citric None Acid Chloroacetate Acid Dried at 105° C.Saturated Capacity (g/g) 12.8 12.3 10.3 17.1 Absorption Pressure (cm) @12.2 8.6 11.5 7.1 50% of Saturated Capacity Desorption Pressure (cm) @24.7 18.1 19.8 14.3 50% of Saturated Capacity Cured at 175° C. SaturatedCapacity (g/g) 11.3 11.2 9.5 14.4 Absorption Pressure (cm) @ 13.0 6.812.6 7.2 50% of Saturated Capacity Desorption Pressure (cm) @ 24.6 18.119.6 13.7 50% of Saturated Capacity

EXAMPLE 21

[0182] Chemical Application

[0183] An aqueous solution of oxalic acid and sodium hypophosphite wasprepared by mixing 151 pounds of a 10% by weight oxalic acid solution,15 pounds of a 50% by weight sodium hypophosphite solution, and 1 poundof water.

[0184] A 10 inch wide roll of Foley Fluff™ fibers, available fromBuckeye Technologies Inc., was unwound and slowly passed through apuddle press. Flooded at the nip of the puddle press was the aqueoussolution of oxalic acid and sodium hypophosphite. The sodiumhypophosphite moderates pulp darkening at high temperature. Through thepuddle press, the sheet absorbed enough of the aqueous solution to reacha moisture content of about 47%, based upon 100% total weight of dryfibers. The treated sheet also contained about 10% by weight of oxalicacid -and 5% sodium hypophosphite, based upon 100% total weight of dryfibers.

[0185] Sheet Disintegration and Fluffing

[0186] Following the puddle press, the sheet was picked apart intosmaller pieces through a shredder, a pre-breaker and a picker. Thedisintegrated pulp was then blown into the inlet of a Sunds DefibratorModel 3784 RO Fluffer, available from Sunds Defibrator, AB of Sundsvall,Sweden, with a gap setting of 5.5 mm. The defibrator fluffed the pulpinto masses of separated fibers. The fluffed pulp was swept out of theRO Fluffer with a high velocity stream of hot air at approximately 380°F.

[0187] Drying and Curing

[0188] The hot air flow that conveyed the fluffed fiber out of the ROFluffer was boosted with a fan through a flash dryer where all of thewater in the fibers was evaporated. The dried pulp fell onto amechanical inlet conveyor forming a low density high bulk “bed” on theconveyor. The fibers were then transported into a Proctor & SchwartzK16476 tunnel dryer, available from Proctor & Schwartz, Inc. of Horsham,Pa. Through a series of hot circulating air flows, the fluffed fiber bedwas heated through three chambers in the dryer. In chamber 1, the bedtemperature reached 330-340° F. In chamber 2, the bed temperatureincreased to 375-385° F. In chamber 3, the bed temperature decreased to355-360° F. After the three heating zones, the fiber bed passed throughone last insulated chamber with no additional heat being added. Thetotal time in the tunnel dryer was approximately 11.5 minutes.

[0189] Baling

[0190] The crosslinked fibers fell off the conveyer from the exit sideof the tunnel dryer into a baler model no. 3445, available from AmericanBaler Company of Bellevue, Ohio, where the material was compressed intobales weighing approximately 85-100 pounds.

[0191] All references cited herein are incorporated by reference. To theextent that a conflict may exist between the specification and thereference the language of the disclosure made herein controls.

What is claimed is:
 1. Cellulose fibers having a median desorptionpressure, as determined in a capillary absorption-desorption cycle, of15 cm or less.
 2. The cellulose fibers of claim 1, wherein the cellulosefibers have a median desorption pressure of 14 cm or less.
 3. Thecellulose fibers of claim 1, wherein wherein the cellulose fibers have amedian desorption pressure of 13 cm or less.
 4. The cellulose fibers ofclaim 1, wherein the cellulose fibers have a median desorption pressureof 12 cm or less.
 5. The cellulose fibers of claim 1, wherein thecellulose fibers have a water retention value of 45 percent or less. 6.The cellulose fibers of claim 5, wherein the cellulose fibers have awater retention value of 38 percent or less.
 7. The cellulose fibers ofclaim 6, wherein the cellulose fibers have a water retention value of 30percent or less.
 8. The cellulose fibers of claim 1, wherein thecellulose fibers are crosslinked.
 9. An acquisition and distributionlayer comprising the cellulose fibers of claim
 1. 10. An acquisitionlayer comprising the cellulose fibers of claim
 1. 11. A distributionlayer comprising the cellulose fibers of claim
 1. 12. An absorbentstructure comprising: (a) a top layer comprising cellulose fibers havinga median desorption pressure, as determined in a capillaryabsorption-desorption cycle, of 15 cm or less; and (b) a bottom layercomprising SAP particles, the second layer being in fluid communicationwith the first layer.
 13. The absorbent structure of claim 12, whereinthe cellulose fibers have a median desorption pressure of 14 cm or less.14. The absorbent structure of claim 13, wherein the cellulose fibershave a median desorption pressure of 13 cm or less.
 15. The absorbentstructure of claim 14, wherein the cellulose fibers have a mediandesorption pressure of 12 cm or less.
 16. The absorbent structure ofclaim 12, wherein the cellulose fibers have a water retention value of45 percent or less.
 17. The absorbent structure of claim 16, wherein thecellulose fibers have a water retention value of 38 percent or less. 18.The absorbent structure of claim 17, wherein the cellulose fibers have awater retention value of 30 percent or less.
 19. An absorbent structurecomprising the cellulose fibers of claim
 1. 20. An absorbent structurecomprising the acquisition and distribution layer of claim
 9. 21. Anabsorbent structure comprising the acquisition layer of claim
 10. 22. Anabsorbent structure comprising the distribution layer of claim
 11. 23. Amethod for preparing cellulose fibers comprising the steps of: (a)refining cellulose fibers to a freeness of from about 300 to about 700ml CSF; and (b) crosslinking the refined cellulose fibers.
 24. Themethod of claim 23, wherein the cellulose fibers to be refined in step(a) are wet lap.
 25. The method of claim 23, wherein step (a) comprisesrefining the cellulose fibers to a freeness of from about 500 to about700 ml CSF.
 26. The method of claim 25, wherein step (a) comprisesrefining the cellulose fibers to a freeness of from about 650 to about700 ml CSF.
 27. The method of claim 23, wherein step (b) comprises: (i)mixing the refined cellulose fibers with a crosslinking agent; and (ii)curing the cellulose fibers in the mixture.
 28. The method of claim 23,wherein step (b) comprises: (i) mixing the refined cellulose fibers witha crosslinking agent; (ii) fluffing the cellulose fibers in the mixture;and (iii) curing the cellulose fibers in the mixture.
 29. The method ofclaim 28, wherein step (b)(iii) comprises drying the cellulose fibersand curing the dried cellulose fibers.
 30. The method of claim 28,wherein curing is performed at a temperature ranging from about 150 toabout 175° C.
 31. Cellulose fibers prepared by the method of claim 23.32. A method of preparing an absorbent structure comprising (a)preparing cellulose fibers by the method of claim 23; and (b)incorporating the cellulose fibers into an absorbent structure. 33.Cellulose fibers crosslinked with at least one crosslinking agentselected from saturated dicarboxylic acids, aromatic dicarboxylic acids,cycloalkyl dicarboxylic acids, bifunctional monocarboxylic acids, andamine carboxylic acids and having a median desorption pressure asmeasured in a capillary absorption-desorption cycle of 25 cm or less.34. The cellulose fibers of claim 33, wherein the saturated dicarboxylicacid has 2 to 8 carbon atoms.
 35. The cellulose fibers of claim 34,wherein the saturated dicarboxylic acid has 2 to 6 carbon atoms.
 36. Thecellulose fibers of claim 35, wherein the saturated dicarboxylic acidhas 2 to 4 carbon atoms.
 37. The cellulose fibers of claim 34, whereinthe saturated dicarboxylic acid is selected from oxalic acid, malonicacid, succinic acid, glutaric acid, adipic acid, pimelic acid, subericacid, and any combination of any of the foregoing.
 38. The cellulosefibers of claim 33, wherein the saturated dicarboxylic acid is asaturated hydroxy carboxylic acid.
 39. The cellulose fibers of claim 38,wherein the saturated hydroxy carboxylic acid has 2 to 8 carbon atoms.40. The cellulose fibers of claim 39, wherein the hydroxy saturateddicarboxylic acid is selected from glycolic acid, tartaric acid, malicacid, saccharic acid, mucic acid, and any combination of any of theforegoing.
 41. The cellulose fibers of claim 33, wherein the aromaticdicarboxylic acid has the formula

wherein R¹, R², R³, and R⁴ independently are hydrogen, hydroxy, C₁-C₄alkoxy, C₁-C₄ amino, halogen, or nitro.
 42. The cellulose fibers ofclaim 41, wherein the aromatic dicarboxylic acid is phthalic acid. 43.The cellulose fibers of claim 33, wherein the cycloalkyl dicarboxylicacid has the formula

wherein R⁶, R⁷, R¹⁰, and R¹¹ are independently hydrogen, hydroxy,halogen, C₁-C₄ alkoxy, C₁-C₄ alkyl, amino, or nitro; and R⁸ and R⁹ areindependently hydrogen, halogen, C₁-C₄ alkoxy, or C₁-C₄ alkyl.
 44. Thecellulose fibers of claim 43, wherein the cycloalkyl dicarboxylic acidis 1,2,5,6-tetrahydrophthalic acid.
 45. The cellulose fibers of claim33, wherein the bifunctional monocarboxylic acid is selected from saltsof a haloacetate, hydroxy monocarboxylic acids, acid derivatives ofhydroxy monocarboxylic acids, and any combination of any of theforegoing.
 46. The cellulose fibers of claim 45, wherein the salt of ahaloacetate is sodium chloroacetate.
 47. The cellulose fibers of claim33, wherein the amine carboxylic acid is an amino acid.
 48. Thecellulose fibers of claim 47, wherein the amino acid has the formulaH₂N—CH₂—R¹²—C(O)OH wherein R¹² is a bond, C₁-C₁₂ alkyl, or C₁-C₁₂ alkylsubstituted with one or more of carboxyl, hydroxy, C₁-C₄ alkoxy, C₁-C₄alkyl, amino, and nitro.
 49. The cellulose fibers of claim 47, whereinthe amino acid has the formula

where R⁵ is a linear or branched C₁-C₈ alkyl.
 50. The cellulose fibersof claim 49, wherein R⁵ is a C₂-C₄ alkyl.
 51. The cellulose fibers ofclaim 47, wherein the amino acid is selected from aspartic acid,glutamic acid, and any combination of any of the foregoing.
 52. Thecellulose fibers of claim 33, wherein the amine carboxylic acid isethylenedinitrilotetraacetic acid.
 53. The cellulose fibers of claim 33,wherein the cellulose fibers have been crosslinked with from about 5 toabout 21 mole percent of crosslinking agent, calculated on a celluloseanhydroglucose molar basis.
 54. The cellulose fibers of claim 33,wherein the cellulose fibers have been crosslinked in the presence of acrosslinking facilitator.
 55. The cellulose fibers of claim 54, whereinthe crosslinking facilitator and the crosslinking agent are different.56. The cellulose fibers of claim 54, wherein the crosslinkingfacilitator is oxalic acid.
 57. The cellulose fibers of claim 54,wherein the cellulose fibers have been crosslinked in the presence offrom about 1.8 to about 9 mole percent of crosslinking facilitator,calculated on a cellulose anhydroglucose molar basis.
 58. The cellulosefibers of claim 54, wherein the cellulose fibers have been crosslinkedwith from about 0.5 to about 40 mole percent of crosslinking agent andcrosslinking facilitator, calculated on a cellulose anhydroglucose molarbasis.
 59. The cellulose fibers of claim 58, wherein the cellulosefibers have been crosslinked with from about 1 to about 30 mole percentof crosslinking agent and crosslinking facilitator, calculated on acellulose anhydroglucose molar basis.
 60. The cellulose fibers of claim33, wherein the cellulose fibers are derived from wood pulp.
 61. Thecellulose fibers of claim 33, wherein the cellulose fibers have beenrefined prior to crosslinking.
 62. The cellulose fibers of claim 61,wherein the cellulose fibers have been refined to a freeness of fromabout 300 to about 700 ml CSF prior to crosslinking.
 63. The cellulosefibers of claim 62, wherein the cellulose fibers have been refined to afreeness of from about 500 to about 700 ml CSF prior to crosslinking.64. The cellulose fibers of claim 63, wherein the cellulose fibers havebeen refined to a freeness of from about 650 to about 700 ml CSF priorto crosslinking.
 65. The cellulose fibers of claim 33, wherein thecellulose fibers have been cured at a temperature of from about 105 toabout 225° C.
 66. The cellulose fibers of claim 65, wherein thecellulose fibers have been cured at a temperature of from about 150 toabout 190° C.
 67. The cellulose fibers of claim 66, wherein thecellulose fibers have been cured at a temperature of from about 160 toabout 175° C.
 68. The cellulose fibers of claim 33, wherein thecellulose fibers have been cured in the presence of a reducing agent.69. The cellulose fibers of claim 68, wherein the reducing agent is ahypophosphite.
 70. The cellulose fibers of claim 69, wherein thereducing agent is sodium hypophosphite.
 71. The cellulose fibers ofclaim 33, wherein the water retention value of the cellulose fibers is50 percent or less.
 72. The cellulose fibers of claim 71, wherein thewater retention value of the cellulose fibers is 45 percent or less. 73.The cellulose fibers of claim 72, wherein the water retention value ofthe cellulose fibers is 38 percent or less.
 74. The cellulose fibers ofclaim 73, wherein the water retention value of the cellulose fibers is30 percent or less.
 75. The cellulose fibers of claim 33, wherein themedian desorption pressure of the cellulose fibers as measured in acapillary absorption-desorption cycle is 20 cm or less.
 76. Thecellulose fibers of claim 75, wherein the median desorption pressure ofthe cellulose fibers as measured in a capillary absorption-desorptioncycle is 18 cm or less.
 77. The cellulose fibers of claim 76, whereinthe median desorption pressure of the cellulose fibers as measured in acapillary absorption-desorption cycle is 15 cm or less.
 78. Thecellulose fibers of claim 33, wherein the crosslinking is substantiallyreversible.
 79. The cellulose fibers of claim 33, wherein thecrosslinking agent is oxalic acid and the crosslinking is substantiallyreversible.
 80. Uncrosslinked cellulose fibers prepared byuncrosslinking the cellulose fibers of claim
 33. 81. The uncrosslinkedcellulose fibers of claim 80, wherein the crosslinking agent contains 4carbon atoms or less.
 82. The uncrosslinked cellulose fibers of claim81, wherein the crosslinking agent is oxalic acid.
 83. The uncrosslinkedcellulose fibers of claim 81, wherein the crosslinking agent is sodiumchloroacetate.
 84. The uncrosslinked cellulose fibers of claim 80,wherein the uncrosslinking step comprises soaking the cellulose fibersin water.
 85. The uncrosslinked cellulose fibers of claim 84, whereinthe uncrosslinking step comprises soaking the cellulose fibers in waterfor from about 0.5 to about 4 hours.
 86. A sheet comprising theuncrosslinked cellulose fibers of claim
 80. 87. An absorbent structurecomprising the fibers of claim
 33. 88. A method of preparing crosslinkedcellulose fibers comprising intrafiber crosslinking the cellulose fiberswith at least one saturated dicarboxylic acid, aromatic dicarboxylicacid, cycloalkyl dicarboxylic acid, bifunctional monocarboxylic acid, oramine carboxylic acid.
 89. The method of claim 88, wherein the saturateddicarboxylic acid has 2 to 8 carbon atoms.
 90. The method of claim 89,wherein the saturated dicarboxylic acid has 2 to 6 atoms.
 91. The methodof claim 90, wherein the saturated dicarboxylic acid has 2 to 4 carbonatoms.
 92. The method of claim 89, wherein the saturated dicarboxylicacid is selected from oxalic acid, malonic acid, succinic acid, glutaricacid, adipic acid, pimelic acid, suberic acid, and any combination ofany of the foregoing.
 93. The method of claim 88, wherein the saturateddicarboxylic acid is a saturated hydroxy carboxylic acid.
 94. The methodof claim 93, wherein the saturated hydroxy carboxylic acid has 2 to 8carbon atoms.
 95. The method of claim 94, wherein the C₂-C₈ hydroxysaturated dicarboxylic acid is selected from glycolic acid, tartaricacid, malic acid, saccharic acid, mucic acid, and any combination of anyof the foregoing.
 96. The method of claim 88, wherein the aromaticdicarboxylic acid has the formula

wherein R¹, R², R³, and R⁴ independently are hydrogen, hydroxy, C₁-C₄alkoxy, C₁-C₄ alkyl, amino, halogen, or nitro.
 97. The method of claim96, wherein the aromatic dicarboxylic acid is phthalic acid.
 98. Themethod of claim 88, wherein the cycloalkyl dicarboxylic acid has theformula

wherein R⁶, R⁷, R¹⁰, and R¹¹ are independently hydrogen, hydroxy,halogen, C₁-C₄ alkoxy, C₁-C₄ alkyl, amino, or nitro; and R⁸ and R⁹ areindependently hydrogen, halogen, C₁-C₄ alkoxy, or C₁-C₄ alkyl.
 99. Themethod of claim 98, wherein the cycloalkyl dicarboxylic acid is1,2,5,6-tetrahydrophthalic acid.
 100. The method of claim 88, whereinthe bifunctional monocarboxylic acid is selected from salts of ahaloacetate, hydroxy monocarboxylic acids, acid derivatives of hydroxymonocarboxylic acids, and any combination of any of the foregoing. 101.The method of claim 100, wherein the salt of a haloacetate is sodiumchloroacetate.
 102. The method of claim 88, wherein the amine carboxylicacid is an amino acid.
 103. The method of claim 102, wherein the aminoacid has the formula H₂N—CH₂—R¹²—C(O)OH wherein R¹² is a bond, C₁-C₁₂alkyl, or C₁-C₁₂ alkyl substituted with one or more of carboxyl,hydroxy, C₁-C₄ alkoxy, C₁-C₄ alkyl, amino, and nitro.
 104. The method ofclaim 102, wherein the amino acid has the formula

where R⁵ is a linear or branched C₁-C₈ alkyl.
 105. The method of claim104, wherein R⁵ is a C₂-C₄ alkyl.
 106. The method of claim 102, whereinthe amino acid is selected from aspartic acid, glutamic acid, and anycombination of any of the foregoing.
 107. The method of claim 88,wherein the amine carboxylic acid is ethylenedinitrilotetraacetic acid.108. The method of claim 88, wherein the mole percent of crosslinkingagent ranges from about 5 to about 21 mole percent, calculated on acellulose anhydroglucose molar basis.
 109. The method of claim 88,wherein the crosslinking step is performed in the presence of acrosslinking facilitator.
 110. The method of claim 109, wherein thecrosslinking agent is different than the crosslinking facilitator. 111.The method of claim 109, wherein the crosslinking facilitator is oxalicacid.
 112. The method of claim 109, wherein the mole percent ofcrosslinking facilitator ranges from about 1.8 to about 9 mole percent,calculated on a cellulose anhydroglucose molar basis.
 113. The method ofclaim 109, wherein the mole percent of crosslinking agent andcrosslinking facilitator ranges from about 0.05 to about 40, calculatedon a cellulose anhydroglucose molar basis.
 114. The method of claim 113,wherein the mole percent of crosslinking agent and crosslinkingfacilitator ranges from about 1 to about 30, calculated on a celluloseanhydroglucose molar basis.
 115. The method of claim 88, wherein thecrosslinking step comprises: (i) mixing the cellulose fibers with thecrosslinking agent; and (ii) curing the cellulose fibers in the mixture.116. The method of claim 115, wherein the crosslinking step comprises:(i) mixing the cellulose fibers with the crosslinking agent; (ii)fluffing the cellulose fibers in the mixture; and (iii) curing thecellulose fibers in the mixture.
 117. The method of claim 116, whereinstep (iii) comprises drying the cellulose fibers and curing the driedcellulose fibers.
 118. The method of claim 115, wherein curing isperformed at a temperature ranging from about 150 to about 175° C. 119.The cellulose fibers of claim 88, wherein the fibers are crosslinked inthe presence of a reducing agent.
 120. The cellulose fibers of claim119, wherein the reducing agent is a hypophosphite.
 121. The cellulosefibers of claim 120, wherein the reducing agent is sodium hypophosphite.122. The method of claim 88, wherein the cellulose fibers are refinedprior to the crosslinking step.
 123. The method of claim 122, whereinthe cellulose fibers are refined to a freeness of from about 500 toabout 700 ml CSF.
 124. The method of claim 123, wherein the cellulosefibers are refined to a freeness of from about 650 to about 700 ml CSF.125. Cellulose fibers prepared by the method of claim
 88. 126. A methodof preparing uncrosslinked fibers comprising the steps of intrafibercrosslinking cellulose fibers with at least one saturated dicarboxylicacid, aromatic dicarboxylic acid, cycloalkyl dicarboxylic acid,bifunctional monocarboxylic acid, or amine carboxylic acid; anduncrosslinking the crosslinked cellulose fibers.
 127. The method ofclaim 126, wherein the crosslinking agent contains 4 carbon atoms orless.
 128. The method of claim 127, wherein the crosslinking agent isoxalic acid.
 129. The method of claim 127, wherein the crosslinkingagent is sodium chloroacetate.
 130. The method of claim 126, wherein theuncrosslinking step comprises soaking the crosslinked cellulose fibersin water.
 131. The method of claim 130, wherein the uncrosslinking stepcomprises soaking the crosslinked cellulose fibers in water for fromabout 0.5 to about 4 hours.
 132. A method of preparing a sheet ofuncrosslinked cellulose fibers comprising the steps of preparinguncrosslinked cellulose fibers by the method of claim 126 and formingthe uncrosslinked cellulose fibers into a sheet.
 133. A method ofpreparing crosslinked cellulose fibers comprising the steps of: (a)preparing uncrosslinked cellulose fibers by the method of claim 126; and(b) crosslinking the cellulose fibers.
 134. A method of preparing anabsorbent structure comprising (a) preparing cellulose fibers by themethod of claim 88; and (b) incorporating the cellulose fibers into anabsorbent structure.
 135. An absorbent core comprising superabsorbentpolymer particles and reversible crosslinked cellulose fibers.
 136. Theabsorbent core of claim 135, wherein the reversible crosslinkedcellulose fibers are crosslinked with oxalic acid, sodium chloroacetate,or a mixture thereof.
 137. The absorbent core of claim 136, wherein thereversible crosslinked cellulose fibers are crosslinked with oxalicacid.
 138. The absorbent core of claim 135, wherein the absorbent corecomprises from about 30 to about 70% by weight of superabsorbentparticles and from about 70 to about 30% by weight of reversiblecrosslinked fibers, based on 100% total weight of the absorbent core.